Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in what is called a projection exposure apparatus, which includes an illumination device and a projection lens. The image of a mask (=reticle) illuminated via the illumination device is projected via the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (e.g. photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
Further optical systems used in microlithography also include inspection systems e.g. in the form of a mask inspection system (for inspecting reticles, or masks, for use in a projection exposure apparatus) or of a wafer inspection apparatus (for observing and testing wafer surfaces). A wafer inspection apparatus has an epi-illumination microscope, the objective of which images the wafer plane onto a TDI sensor or a camera, wherein the image data are examined numerically for deviations from a predetermined image to check the microlithographic imaging result.
Various approaches have been developed both for the illumination device and for the lens of a microlithographic projection exposure apparatus or the objective of an inspection microscope in order to selectively influence the polarization state and the wavefronts of the eigen-polarizations and the difference between them, what is referred to as retardation, or to compensate for existing disturbances.
However, in practice, the problem occurs that an initially set polarization state or a retardation can be changed in an undesired manner. The responsible influences include in particular time-variable birefringence effects such as what is known as the polarization-induced birefringence (PDB), birefringence as a result of time-variable stresses, compaction in non-crystalline material (e.g. quartz glass) of optical components, degradations and thermal effects and also birefringence, present in anti-reflective or highly reflective layers on the optical components, as a result of form birefringence or due to different Fresnel reflection and transmission for orthogonal polarization states.
Merely exemplary background references include DE 10 2007 055 567 A1 and DE 10 2008 040 613 A1.